Patent GB-883360-A (GB)
Title: Improved magnetic tape recording and/or reproducing apparatus
IPC: H04N5/782
Filed: 1959-07-14
Abstract: 883,360. Automatic speed-control systems. AMPEX CORPORATION. July 14, 1959 [Aug. 4, 1958], No. 24229/59. Class 38 (4). Magnetic tape recording and reproducing apparatus suitable for colour television signals comprises a tape 11 which is driven past a transducing head assembly by means of a capstan 13, a plurality of heads 16 being carried on disc 18 driven by synchronous motor 19. A guide 21 serves to cup the tape as it is drawn past the transducing units, the tape being in continuous pressure contact with the heads as they sweep through a circular path. A desired relationship between the speeds of the disc 18 and capstan 13 is maintained by recording a control signal on the tape, using a magnetic transducing head 31. A standard frequency source 36 is connected to a frequency divider 37 to derive a signal which is applied to amplifier 38 driving the motor 19. This motor is coupled to disc 39 painted black and white and co-operating with light 41 and photo-cell 42. The output frequency of the cell indicates the speed of the motor 19, and the square-wave derived from the cell is passed through shaper 43 to frequency divider 44. During the recording operation, the output of band-pass filter 46 is applied to amplifier 47 and is employed to drive the capstan motor 48. The output from the shaper is also applied through filter 49 to amplifier 51 which supplies control track head 31. During reproduction, the timing reference signal is again applied to divider 37, amplified, and fed to the motor 19. The signal from filter 49 is fed to a phase comparator and servo amplifier 52, a second signal being applied to the comparator from amplifier 53 receiving the output signal from the control track. The resultant signal is fed to amplifier 47 for controlling the speed of the capstan motor whereby the tape is advanced a predetermined distance during each revolution of the disc 18. The instantaneous angular velocity of the head is measured by projecting light from source 54 on to tachometer disc 56. The reflected light is intercepted by photo-sensitive element 57 connected to amplifier 58. The amplified signal is applied to phase detector 59 and is compared with a reference signal derived from source 36 through frequency multiplier 36 and amplifier 62. The error signal from the phase comparator is applied to frequency-sensitive network 63 the output of which is applied through amplifier 64 to eddy-current brake 66. Thus, the load on the motor is varied in response to the error signal to maintain a constant angular velocity. The feedback loop is designed to reject signals corresponding to the mechanical resonance of the system. The functions of the circuit elements 58, 59, 61 and 62 are performed by a circuit comprising a two-stage transistor amplifier receiving the signal from the photo-sensitive element 57, the tuned collector load of the second transistor being shunted by the base circuit of a third transistor which acts as a phase detector, Fig. 10 (not shown). The reference signal is half-wave rectified by a diode in the base circuit of a transistor which amplifies the fourth harmonic selected by the resonant collector load, and has its collector capacitively coupled to a further transistor acting as a tuned amplifier. The driving signal for the amplifier 38 may be developed by a phase-controlled oscillator to which is applied a signal of reference frequency, the oscillator comprising a transistor connected as a phase detector as well as one connected as an amplifier and serving to receive the output signal from the phase detector. The output of a transistor-oscillator circuit is applied to the phase detector for comparison with the reference signal, the amplified output of the phase detector controlling the frequency of oscillation of the oscillator, Fig. 11 (not shown). In another arrangement, the apparatus 37 comprises a valve circuit in which a signal frequency is amplified and passed to a mixer tuned to a fraction of this frequency, the fractional frequency being multiplied and fed back to the first mixer. A second mixer has an output circuit tuned to a lower fractional frequency which is multiplied and fed back to the second mixer, a third mixer receiving both fractional frequencies and having an output frequency equal to the difference therebetween, Fig. 12 (not shown). A suitable servo amplifier and filter network 63, 64 comprises an amplifier having two channels, the first amplifying D.C. to a few cycles and the other amplifying signals having a frequency above the mechanical resonance of the system. The output from both channels is combined and applied to the eddycurrent brake, Fig. 13 (not shown). Specifications- 798,927, 798,928, 798,929 and 798,930 are referred to.